In general, a conductor roll (1) is an electric apparatus for plating a steel sheet, installed in electrolytic bath, which is immersed into the plating solution (L) with Zinc (Zn) or Nickel (Ni) as an anode, and the conductor roll itself as a cathode. By continuously inducing a steel sheet (S) in this manner, the conductor roll carries out electroplating as illustrated in FIG. 1.
Here, with respect to the conductor roll (1), if the entire portion of its surface consists of a conductive material, the roll itself becomes electroplated. As such, in order to plate the steel sheet while not plating the conductor roll in itself, the conductor roll (1) which is in direct contact with the steel sheet (S) is made out of a non-conductive material at the outer sections (d) of the conductor roll (i.e., both end portions of the cylinder).
In other words, as illustrated in FIG. 2, a conductor roll (1) comprises an inner section (D) in the central portion of the cylinder, and an outer section (d) at the either end portion of said inner section (D). The inner section (D) comprises a conductive material, such as steel or a metal material, having superior acid and corrosion resistances in a strongly acidic plating solution. The outer section (d) comprises a non-conductive material such as rubber. Meanwhile, the width of the conductive metal material at the inner section (D) of the conductor roll (1) should be less than the minimal width of the steel sheet (S) to be plated (i.e., generally lesser by 100 mm).
On the other hand, Japanese Patent No. 10,245,695 (Sep. 14, 1998), as a means of enhancing corrosion and wear resistances of a conductor roll, teaches a method of spray-coating the steel band portion with a mixture of nickel-based alloy and tungsten carbide (WC). Meanwhile, Japanese Patent No. 4,346,693 (Dec. 2, 1992) teaches a method of enhancing corrosion and wear resistances by coating the cobalt or nickel-based alloy with ceramics of relatively superior electric conductivity (to the degree of carbides).
Moreover, as for the non-conductive material used in the outer section (d), which comprises the both end portions of the conductor roll (1), ebonite (i.e., polymer materials), multilastic, sponge, polyurethane, etc. are used.
Moreover, as for the plating solution for immersing the conductor roll (1) during the electroplating process, the temperature used therein is approximately 70° C. Due to the differences in the mutual thermal expansion coefficients as between the metal band portion (3) of the inner section (D) and the rubber section (4) of the outer section (d) at the temperature of the plating solution in use, the rubber section (4) is characterized by greater expansion as compared to the metal band portion (3).
Therefore, in consideration of the thermal expansion coefficient of the metal band portion (3) of the conductor roll (1) and that of the rubber section (4), the level difference (h) of the rubber section (4) is made in such a manner to be lower than the level difference of the metal band portion (3). In this manner, the level of the metal band portion (3) of the inner section (D) and that of the rubber section (4) of the outer section (d) are maintained at a certain level at the temperature of plating solution in use (approximately 70° C.).
However, if the deflector roll (2), which pulls the plating metal sheet (S) in both directions, applies tension via the conductor roll (1), the plating metal sheet presses against the rubber section (4), which in turn results in deformation of the rubber section (4). In particular, depending on the quality of the material, the thickness thereof, and the degree of tension therein, the extent to which it presses against the rubber section (4) may vary.
In other words, as shown in FIG. 2, as the load of the plating steel sheet (2) becomes directly concentrated on the ends of the metal band portion (3), it induces a curve deformation (W), more severe than the level difference thereof (h). Consequently, a strip is formed (i.e., band mark) on the surface of the steel sheet (S) passing under the curve deformation region (W). This type of a band mark could be clearly confirmed with the naked eye and is one of the most severe defects of the plating steel sheet.
If the level difference (h) between the metal band portion (3) and the rubber section (4) is made to be smaller in order to reduce the band mark on a steel sheet (S), the load of the steel sheet becomes concentrated on the rubber section, and the frequency of occurrence of band marks is decreased. However, a gap is created between the conductive metal band portion and the plating steel sheet. Consequently, as shown in FIG. 4(a), static electricity (arc) is created therein.
Moreover, if static electricity is generated between the metal band portion and the plating steel plate, the metal band portion and the rubber section rapidly become damaged by static electricity. When this type of situation occurs, an abrasion work on the conductor roll must be immediately carried out once again.
Consequently, if the level difference (h) between the metal band portion (3) and the rubber section (4) is made to be smaller, the frequency of occurrence of band marks is reduced although there is a problem associated with the reduction of life of the conductor roll due to the premature damage to the metal band portion and the rubber section.
In this regard, in consideration of these problems caused by the aforementioned structural defects of a conductor roll, the present invention was devised with an objective of providing a means of eliminating a band mark on a steel sheet, which uses a non-conductive ceramic material, instead of a polymer material with a large thermal expansion coefficient.
In achieving the aforementioned objective, the present invention comprises ceramic coating portions of circular bands, placed respectively in a thin strip at the both edge regions of the metal band portion at the central portion of a conductor roll. As such, the present invention has the effects of suppressing the occurrence of static electricity and also reducing a band mark on an electroplating steel sheet by eliminating the level difference between the conductive material (metal band portion) and the non-conductive material (rubber section) thereof. Furthermore, the present invention is capable of extending the life of a conductor roll by enhancing the wear and corrosion resistances thereof.